This invention relates to a drive shield for use in a tunnelling operation, and in particular to a drive shield for use in pipe laying. The term "tunnel" or "tunnels" used throughout this specification is intended to include galleries, trenches, adits and the like elongated excavations.
In a known pipe-laying operation, which is particularly useful for laying pipes such as sewers, the pipe sections are thrust-jacked, from the rear of the pipe, along a tunnel by means of a hydraulic jacking station. A drive shield having a wedge-shaped cutting edge is used to excavate the tunnel, at the front of the pipe, in conjunction with a cutter/loader contained therein. When laying long pipes, it may be necessary to provide intermediate jacking stations along the pipe's length. This drive shield is provided with a thrust ring, which abuts the front end of the pipe, and with hydraulic advance rams interposed between the thrust ring and the drive shield for advancing the drive shield. Not only do these hydraulic advance rams constitute the means for advancing the drive shield relative to the laid pipe, but they also provide a certain degree of steering control to this advance movement.
The main disadvantage of this known drive shield is that, owing to the extraordinarily high jacking forces, pear-shaped pressure zones are formed in the region in front of the cutting edge. In the event of a sudden pressure release, the ground in these regions tends to collapse and fill up the interior of the drive shield. Another disadvantage is that, if the cutting edge hits an obstacle such as rock, the drive shield may be deflected and forced to advance in the wrong direction.